The present invention relates to a method for measurement of the radar target cross section of an object with both moving and fixed parts. One conceivable such object are jet aircraft, in which the moveable parts primarily consist of moving parts of the jet engine, and the fixed parts of the object primarily consist of the aircraft fuselage.
In theoretical calculations of the radar target cross section of an object it can be very difficult to take proper account of the effect of parts of the object that move in operation. An example of objects with parts that move in operation are jet aircraft, in which the moving parts consist chiefly of the so-called blades in the various compressor stages of the jet engine.
In measurements of the radar target cross section of a jet aircraft in operation, so-called dynamic measurement, the moving parts will generate modulations in the signal received. The said modulations are of a nature such that they make it more difficult to extract data from the measurements concerning the radar target cross section of other parts of the aircraft, since the dispersion and radar target cross section that derive from the said modulations are very difficult to calculate or measure with stationary measuring equipment.
There is a need therefore, when measuring the radar target cross section of an object with both moving and fixed parts, for a method of distinguishing the modulations in the signal received that derive from moving parts of the object and from fixed parts of the object respectively.
The present invention solves the problem, when measuring the radar target cross section of an object with both moving and fixed parts, of being able to distinguish the modulations in the signal received that derive from moving parts of the object and from fixed parts of the object respectively.
This problem is solved by a method that comprises a first measurement with high frequency resolution, from which information can be extracted on the modulations of the signal received that derive from moving parts of the object and from fixed parts of the object respectively, and a second measurement with high range resolution. The measuring result obtained from the second measurement is filtered around a certain frequency, which is obtained by means of the measuring result from the first measurement.
Since the first measurement has good frequency resolution it is possible to extract information on modulations of the signal received that derive from moving parts of the object and from fixed parts of the object respectively. The fact that the second measurement has high resolution in the range direction means that the measuring result from the second measurement can be used to determine the target cross section of different parts of the object.
Owing to the moving parts in the object, data from the measurements will contain modulations at a large number of frequencies. The frequency of the modulations in the second measurement that derive from that part of the aircraft, the target cross section of which it is wished to calculate, is obtained by means of measuring results from the first measurement, following which the desired measuring results from the second measurement can be filtered out and used for further processing, for example in order to distinguish target cross sections that derive from moving parts from target cross sections that derive from fixed parts of the object.